Video and still image capture using a digital camera has become very prevalent. Video capture may be used for such applications as video conferencing, video editing, and distributed video training. Still image capture with a digital camera may be used for such applications as photo albums, photo editing, and compositing.
Digital cameras are typically expected to operate under a variety of scene illuminations. Some common illuminant conditions are fluorescent lighting, daylight, and tungsten illumination. These light sources have different spectral (wavelength) components, as shown in FIG. 1. For example, the tungsten light source has a stronger red wavelength component (approximately 600 nm–700 nm) compared to the blue wavelength component (approximately 400–500 nm), and green wavelength component (approximately 500–600 nm). In comparison, fluorescent lighting has stronger blue and green wavelength components with a relatively weaker red wavelength component.
Many digital video and still image capture systems use an image sensor that is constructed from a complementary metal oxide semiconductor (CMOS) process. CMOS technology offers the ability to integrate signal processing circuitry directly onto the sensor to achieve a lower system cost or to enable unique functionality within the sensor itself. The image sensing portion of a CMOS sensor is constructed of an array of light sensitive elements, each commonly referred to as a “pixel” element. Each pixel element is responsible for capturing one of three color channels: red, green, or blue. Specifically, each pixel element is made sensitive to a certain color channel through the use of a color filter placed over the pixel element such that the light energy reaching the pixel element is due only to the light energy from a particular spectrum. Each pixel element generates a signal that corresponds to the amount of light energy to which it is exposed.
The charge collected by each pixel element to form an image is determined by the illuminant's spectral energy components, the content of the scene, and the digital camera's photoresponsivity. To achieve the goal of reproducing the scene content for the user, the camera must remove the effects of the illuminant spectral components and the imaging module photoresponsivity. The camera photoresponsivity is stable and can be characterized during design or manufacturing. The scene illuminant, however, is variable and must be compensated for each time a picture is taken. Imaging across varying illuminant conditions requires careful exposure settings to capture good images in spite of the lighting component strength variability. Part of the challenge is in obtaining sufficient signal to noise ratio in the signals captured for each color channel of the camera. Color channel signal to noise ratio is fundamental to the final image color balance and accurate reproduction of color and tone under different illuminating conditions. Once the type of illuminant is determined through the use of methods such as flicker frequency (or “harmonics”) detection (for detecting fluorescent lighting), pre-metering white balance (automatic gain control), or statistical analysis of sample data, then the correct exposure controls and algorithms can be applied ot obtain accurate colors in images.
Currently, image capture using sensors is such that all three color channels have the same integration time. With the same integration time being used for all color channels, captured images often result with many of the pixel elements of one color channel being saturated or, alternatively, having little signal, depending on the chosen integration time and the wavelength distribution for the illuminant. Thus, it is desirable to be able to control the integration of each color channel independently such that charge accumulation levels for the pixel elements of each color channel may be set independently.